1. Field of the Invention
The present invention relates to an electrophotographic image formation method using a two-component developer, a carrier for the two-component developer, the two-component developer for use in electrophotography, a container holding therein the two-component developer, and an electrophotographic image formation apparatus.
2. Discussion of Background
Conventionally, many electrophotographic image formation methods are known. Generally, in those electrophotographic image formation methods, a latent electrostatic image is formed on a photoconductor containing therein a photoconductive material, utilizing the characteristics of the photoconductive material, by use of various means. The latent electrostatic image is then developed with a developer to a visible toner image, and when necessary, the visible toner image is transferred to a transfer sheet such as a sheet of paper and fixed thereto with the application of heat and/or pressure or the like thereto, whereby a copy is obtained.
Recently, in addition to the conventional copying machines, there have been used a large number of apparatus, using eiectrophotography, such as printers and facsimile apparatus. In the field of copying machines and printers, there is always customer""s demand for high speed and stabilized operation. Currently in such high speed copying machines and printers, a two-component development method, using a two-component developer comprising a carrier and a toner is mainly used.
In the current course of development of such high speed copying machines and printers, one of the largest subjects to be targeted is to secure stabilized development performance with high efficiency.
As one of the trials for the achievement of the above subject, it has been proposed to minimize a gap between a latent image bearing member and a developer bearing member, thereby intensifying an electric field for development, and improving the development performance of toner.
However, an analysis of the above-mentioned trial, conducted by the inventors of the present invention, indicated that as the gap is narrowed, a developer tends to build up between a development area and a member called xe2x80x9cdoctor bladexe2x80x9d for controlling the amount of the developer on the developer bearing member, so that the movement of the developer tends to slow. When a development bias charge is applied to the developer in such a state, electric charges are selectively injected into the carrier when the carrier has a low resistivity. The result is that instead of the toner, the carrier is deposited on an image area on the latent image bearing member, so that the latent electrostatic image in the image area is developed, not with the toner, but with the carrier. When such deposition of the carrier takes place on the latent image bearing member, the deposited carrier works as a spacer between the image transfer material and the latent image bearing member. As a result, non-image-transferred portions are formed in the transferred image area on the image transfer material.
There have not yet been devised any countermeasures against such deposition of the carrier on the latent electrostatic image bearing member, which takes place when the gap between the latent electrostatic image bearing member and the developer bearing member is narrowed.
However, there has been a conventional proposal of increasing the electric resistivity of the carrier itself to about 1013 xcexa9xc2x7cm or more, without being aware of narrowing the gap between the latent electrostatic image bearing member and the developer bearing member. For example, in Japanese Laid-Open Patent Application No. 7-234548, it is proposed that the surface of a core material be coated almost in its entirety with a resin with high resistivity, whereby a carrier with high resistivity can be obtained. In fact, this method produces the effect of reducing the deposition of the carrier. However, this method has the problem that the development performance is reduced so that images with sufficiently high density for practical use cannot be obtained.
It is therefore a first object of the present invention to provide an electrophotographic image formation method by which the above-mentioned conventional problems are solve and which is capable of forming images with high quality, with high development performance, free of the carrier""s deposition and abnormalities in image quality such as the formation of non-image-transferred portions.
The second object of the present invention is to provide a carrier for a two-component developer for use in the above-mentioned electrophotographic image formation method.
The third object of the present invention is to provide a two-component developer for use in the above-electrophotographic image formation method.
The fourth object of the present invention is to provide a container in which the above-mentioned two-component developer is held.
The fifth object of the present invention is to provide an electrophotographic image formation apparatus for use in the above-mentioned electrophotographic image formation method.
The first object of the present invention can be achieved by an electrophotographic image formation method for developing a latent electrostatic image formed on a latent image bearing member with toner to a toner image, using a two-component developer comprising a carrier and a toner, the toner being held on a developer bearing member, with a gap between the mutually facing surfaces of the latent image bearing member and the developer bearing member being set in a range of 0.1 mm to 0.5 mm, and the carrier comprising carrier particles, each carrier particle comprising a core material and a coating material which is coated on the surface of the core material, the coating material comprising a polysiloxane resin comprising at least an oxygen atom (O) and a silicon atom as constituent elements therefor, with an atomic ratio of the constituent elements, O/Si, being in a range of 2.1 to 4.0, and the carrier having a specific resistivity of 109 to 1016 xcexa9xc2x7cm.
The polysiloxane resin used in the carrier for the two-component developer for use in the above electro-photographic image formation method may further comprise a nitrogen atom (N), with an atomic ratio of the nitrogen atom to the silicon. atom, N/Si, being in a range of 0.1 to 4.0.
It is preferable that the carrier have a fluidity of 20 sec/50 g to 40 sec/50 g.
The second object of the present invention can be achieved by a carrier comprising carrier particles, each carrier particle comprising a core material and a coating material comprising a polysiloxane resin, which is coated on the surface of the core material, the polysiloxane resin comprising at least an oxygen atom (O) and a silicon atom as constituent elements therefor, with an atomic ratio of the constituent elements, O/Si, being in a range of 2.1 to 4.0, the carrier having a specific resistivity of 109 to 1016 xcexa9xc2x7cm.
The polysiloxane resin used in the carrier may further comprise a nitrogen atom (N), with an atomic ratio of the nitrogen atom to the silicon atom, N/Si, being in a range of 0.1 to 4.0.
It is preferable that the carrier have a fluidity of 20 sec/50 g to 40 sec/50 g.
The third object of the present invention can be achieved by a two-component developer which comprises a toner and the above-mentioned carrier.
The fourth object of the present invention can be achieved by a container holding therein the above-mentioned two-component developer.
The fifth object of the present invention can be achieved by an electrophotographic image formation apparatus comprising the above-mentioned container.
The fifth object of the present invention can also be achieved by an electrophotographic image formation apparatus comprising a latent electrostatic image bearing member and a developer bearing member, which are disposed with a gap of 0.1 to 0.5 mm between the facing surfaces of the latent electrostatic image bearing member and the developer bearing member.